1. Field of the Invention
This invention relates to a lead-free plain bearing and a method for its manufacture.
2. Description of the Related Art
Plain bearings are bearings that provide sliding contact between opposing surfaces of the bearing and the object being supported by the bearing. Plain bearings are used in a wide variety of equipment as journal bearings, thrust bearings, and guide bearings. Plain bearings are also extensively used as or as part of swash plates in swash plate pumps.
Swash plate pumps, also referred to as in-line axial piston pumps, are widely used as compressors in automotive air conditioning systems and as pumps for hydraulic systems of construction equipment on account of their compactness and efficiency. Such pumps typically include a swash plate and a plurality of cylinders parallel to a common axis, with each cylinder containing a piston directly or indirectly in sliding contact with the swash plate. The swash plate is at an angle to the axis, and the sliding contact between the swash plate and the pistons will cause the pistons to reciprocate within the cylinders to perform a pumping action.
Swash plate pumps may operate at very high speeds (frequently on the order of 8000 rpm), and an extremely high load may be applied to the surface of the swash plate by the pistons, so this surface must have excellent bearing properties, i.e., a high seizing pressure and a low coefficient of friction.
A typical swash plate has a multilayer structure in which a thin layer of a bearing alloy is joined to a backing in the form of a disk-shaped steel plate. The steel backing plate provides the swash plate with structural integrity and decreases the cost thereof, while the bearing alloy forms a bearing surface having the desired bearing properties. If a swash plate were made entirely of a bearing alloy, it would generally have inadequate strength for use at high rotational speeds under high loads, and since a bearing alloy usually has copper as a main component, such a swash plate would be prohibitively expensive.
In general, three different methods have been used for the manufacture of multilayer swash plates. These are the pressure bonding method, the melting method, and the sintering method.
In the pressure bonding method, a plate made of a bearing alloy is superimposed on a steel backing plate, and the two plates are subjected to rolling to bond the plates to each other. However, the bond between the two plates is not extremely strong, so when a swash plate formed by this method is used in large hydraulic equipment, there are cases in which peeling occurs, so the swash plate may have a short lifespan.
In the melting method, a molten bearing alloy is cast onto a steel backing plate and is cooled until it solidifies and is joined to the backing plate. The solidified bearing alloy has oxides remaining on its surface, and due to shrinkage occurring during solidification, the surface of the bearing alloy may become uneven, so it undergoes grinding to smooth it. Due to the abrupt heating of the steel backing plate by the molten alloy, there are cases in which its surface oxidizes or undergoes deformation due to thermal strains, so the backing plate is also subjected to grinding to smooth it. The multilayer structure which is obtained in this manner is punched with a press or cut with a laser to give it a desired shape, such as circular. Thus, the melting method is not only complicated due to the large number of steps it involves, but inclusion of oxides can take place during pouring and shrinkage cavities can be formed during solidification, so there can be wide variation in bearing properties with this method.
In the sintering method, a bearing alloy powder is uniformly dispersed to a prescribed thickness on a disk-shaped steel backing plate. The backing plate and the bearing alloy powder are then heated in a furnace in a hydrogen atmosphere to sinter the grains of the powder to each other and to the backing plate. At the completion of sintering, the grains of powder retain their granular shape, so the surface of the swash plate is extremely rough, like a file. The swash plate cannot be used in this condition, since the surface formed by the bearing alloy would grind away those portions of pistons or other members contacted by it. Therefore, in order to finish the bearing surface as well as to adjust its thickness, the bearing surface formed by the bearing alloy is subjected to machining with a lathe. The sintering method involves fewer steps than the melting method, and the joint strength between the bearing alloy and the backing plate is strong, so it is generally superior to the other two manufacturing methods.
The bearing alloy used for a conventional swash plate is LBC-3, which is an alloy containing 10 mass percent of Sn, 10 mass percent of Pb, and a remainder of Cu. It has extremely good bearing properties, so a swash plate including this bearing alloy can operate for long periods of time without seizing and with little wear either of the swash plate itself or of pistons or other member in contact with the swash plate. For these reasons, LBC-3 is presently the most widely used material for the bearing surface of swash plates.
When a swash plate is manufactured by the sintering method using LBC-3, powder of LBC-3 is uniformly dispersed atop a steel backing plate, and sintering is then carried out by heating in a hydrogen atmosphere in a furnace maintained at 800xc2x0 C. to obtain a multilayer member having a bearing surface. The multilayer member is refined by pressing with a 150-ton press, and then annealing is carried out in a hydrogen atmosphere in a heating furnace at 800xc2x0 C. in order to remove working strains and work hardening produced by the refinement. After annealing, finishing is performed with a cutting machine to obtain a roughness of approximately 0.5 xcexcm Ra.
Despite the good bearing properties of LBC-3, there has developed a need for a replacement for this material due to potential environmental problems. Although many metal portions of discarded automobiles or construction equipment are capable of being recycled, a swash plate with a multilayer structure generally cannot, since the bearing layer of the swash plate, usually containing a large amount of copper, is tightly joined to the steel backing plate of the swash plate, and the two cannot be readily separated from each other. If such a swash plate is simply melted with steel scrap, the copper in the bearing layer contaminates the steel and makes it totally unusable. Therefore, swash plates are usually disposed of by burial in landfills.
However, if a swash plate in a landfill is contacted by acid rain, the rain can eluate lead from LBC-3 or other lead-containing bearing alloys used in swash plates, and underground water containing such lead can end up contaminating drinking water supplies and creating the threat of lead poisoning. As a result, both the automotive industry and the construction equipment industry now desire a swash plate which does not contain lead, i.e., a lead-free swash plate.
Despite the obvious need for a replacement for LBC-3 and other lead-containing bearing alloys, lead has been considered indispensable as a solid lubricant in bearing alloys for bearings used under severe conditions such as the high loads and high rotational speeds which occur in swash plate pumps, so it has not been possible to simply omit lead from bearing alloys.
This invention provides a lead-free plain bearing which has properties equivalent to or better than those of plain bearings employing LBC-3 as a bearing alloy.
This invention also provides a method of manufacturing such a lead-free plain bearing.
The present inventors investigated the bearing properties of bearing alloys having copper as a main component and containing various metals other than lead as a solid lubricant. It was found that if a suitable amount of silver is added to a main component of copper, bearing properties roughly the same as LBC-3 are obtained. It was also found that the bearing properties of such a silver-containing bearing alloy can be further improved by the addition of one or both of molybdenum disulfide powder and graphite powder. It was further discovered that the surface roughness of a bearing alloy is an important factor in decreasing the initial wear of a plain bearing, i.e., the wear at the initial stage of use of the bearing.
Thus, according to one aspect of the present invention, a lead-free plain bearing comprises a backing and a powder consisting essentially of 7-13 mass percent of Sn, 0.1-5 mass percent of Ag, and a remainder of Cu sintered to the backing.
According to another aspect of the present invention, a lead-free plain bearing comprises a backing and a powder consisting essentially of 7-13 mass percent of Sn, 0.1-5 mass percent of Ag, at least one of 0.05-0.5 mass percent of molybdenum disulfide and 0.1-2 mass percent of graphite powder, and a remainder of Cu sintered to the backing.
According to yet another aspect of the present invention, a method of manufacturing a plain bearing includes dispersing a lead-free bearing alloy powder on a backing and sintering the powder to the backing to form a bearing surface on the backing. In a preferred embodiment, the method further includes refining the bearing surface by pressing and then annealing to remove working strains induced by the pressing. In order to give the bearing surface a desired smoothness, the method will generally include rough finishing of the bearing surface to achieve a surface roughness of 0.3-1 xcexcm Ra, and fine finishing by polishing to a roughness of at most 0.1 xcexcm Ra.
The backing frequently comprises a ferrous member, such as an iron or steel plate, having a shape suitable for the intended use of the bearing. For example, when the bearing is to be used as a swash plate, the backing will frequently be a steel disk. When the bearing is to be used as a journal bearing, the backing will typically be a steel plate capable of being formed into a cylinder.
A plain bearing according to the present invention can be used in a variety of different ways. For example, it can be used as or as part of a swash plate, as a journal bearing, as a thrust bearing, or as a guide bearing.
A lead-free plain bearing according to the present invention can exhibit bearing properties comparable to or superior to those of bearings employing LBC-3, even when used in severe operating conditions, such as in an automotive air conditioner compressor or in construction equipment. In particular, it has the excellent mechanical properties that there is no occurrence of damage or seizing of the bearing or members in sliding contact therewith over long periods of time. As a plain bearing according to the present invention does not contain lead, it can be disposed of by burial underground without causing environmental pollution. A manufacturing method for a plain bearing according to the present invention can reliably secure a bearing alloy to a backing, and it can make the initial wear of the bearing extremely small, resulting in a plain bearing which is ideal for use as a swash plate, for example.